On Sept. 10, Kamala Harris and Donald Trump faced off in their first and possibly only presidential debate. It was the first time two current or former presidents and vice presidents faced off in a presidential campaign.
Harris was the prediction market favorite going into the debate. She led Trump 73 to 27 on Polymarket and 74 to 26 on Manifold. However, Trump’s unpredictability and simple delivery made it possible for him to win. Had he remained on message, he could’ve delivered talking points designed to be easily consumed in soundbites.
Instead, Harris beat Trump, and it wasn’t close. Conservative lawyer, Doug Schoen, wrote an op-ed on Fox News acknowledging Harris’ debate victory. Harris’ strong evening also ended with an endorsement from Taylor Swift after the debate.
How Prediction Markets Saw Harris Coming
Most of the money on the debate winner entered the market on the day of the debate. The amount of money in Polymarket’s presidential debate winner market almost doubled from the morning of Sept. 9 to the morning of Sept. 10. It did double between the morning of the debate and the minutes before the debate began. The money in the market nearly doubled again by the time the debate ended.
| Day Before Debate | Morning of Debate | ~15 Minutes Pre-Debate | Post-Debate | |
|---|---|---|---|---|
| Bet Volume | $114k | $224k | $462k | $743k |
The rapid influx of money leaned toward Harris, the safer bet in the market. The price of her victory rose to 80 cents while Trump’s fell to 23 cents. After a brief period of what were likely sell-offs from traders getting their safe positions out, the market hovered around a 75/25 split favoring Harris.
Those prices held during the first 15 minutes of the debate. Both candidates had focused opening statements that balanced defenses of their policy visions and attacks on their opponents. The first issue was about the economy, where Trump largely stayed on message about inflation and the price of goods. Harris and Trump sparred, but neither noticeably outshone the other.
Trump Surrendered and Harris Filled the Void
Near the 15-minute mark, Trump was asked about abortion. Trump not only defended Roe v. Wade’s overturn, one of his least popular legacies. He also lied about executions of babies shortly after birth. He was out of touch with voters’ concerns about state-by-state abortion policy and surrendered time that could have been spent arguing in favor of states’ rights.
Harris began her answer to abortion policy at about the 19-minute mark in the debate. Her answer included a litany of horror stories resulting from restrictive abortion policies. They included women who bled in their cars waiting for medical care from doctors worried about breaking the law and children forced to carry pregnancies to term. She also pledged to sign a bill reinstating Roe v. Wade’s protections.
Harris’ price of victory ticked up three cents around the time Trump began his answer about abortion. Trump used his rebuttal to talk about the Biden administration’s failed student loan forgiveness policy. By the time he finished, Harris’ victory was trading at 85 cents and Trump’s was down to 16.
Harris increased the spread on Polymarket’s debate market by 18 points in less than five minutes.
Her ability to combine moving stories, devastating attacks, and visions of her own policies made her answers consistently powerful. In contrast, Trump fell back on the same talking points about crime and immigration.
He made points that are likely to resonate with his base, but he pivoted from his role in the Jan. 6 attack on the Capitol and his stance on Russian aggression in Ukraine by talking about immigrant crime. Harris filled the void with answers about her own beliefs and more attacks on Trump.
Trump’s lack of organization and belligerence – including spars with the moderators – contrasted unfavorably with Harris’ poise and focused answers. Her price of victory hit 90 cents a half-hour into the debate and never fell below it afterward.
To top a strong evening off, Harris earned Taylor Swift’s endorsement 45 minutes after the debate. Swift cited Harris’ debate performance and Trump’s AI-generated Swift endorsement as reasons to publicly support Harris’ candidacy. By then, Harris’ price of victory was above 96 cents. Prediction markets tracked voter sentiment in real time and saw Harris’ prosecutorial career bring her through a vital campaign milestone.
